Method to eliminate dissimilar metal welds

ABSTRACT

A method of eliminating dissimilar metal welds has been disclosed. The method includes the steps of providing a first part having a first alloy composition; providing a second part having a second alloy composition different from the first part; connecting a containment structure to the first part; pouring a powder into the containment structure such that the powder is in contact with the first part; positioning a portion of the second part in the containment structure such that the second part compresses the powder between the first and second parts; and performing hot isostatic pressing (HIP) to consolidate the powder and join the first and second parts together.

BACKGROUND OF THE INVENTION

This invention relates generally to a method of joining dissimilarmetals, and more particularly to a method of eliminating dissimilarmetal welds.

For purposes of discussion, the invention will be described withreference to eliminating dissimilar metal welds (DMWs) fornozzle-to-safe end applications, such as those employed in nuclear andfossil plants; however, it should be appreciated that the invention isnot limited to nozzle-to-safe end applications and is applicable to theelimination of DMWs in other applications.

Joining nozzle-to-safe end dissimilar materials has always beenaccomplished using a welding process. However, nozzle-to-safe end weldsare often difficult to produce since they require the use of multipleweld filler metals and an intermediate post weld heat treatment (PWHT)during their assembly. The weld joint is commonly comprised of an SA508low alloy steel nozzle joined to a 304L or 316L stainless steel safeend. Due to the differences in the material compositions, the nozzle isfirst machined to the appropriate weld bevel and a nickel-based alloysuch as Alloy 82 is applied to the face of the bevel. This is oftenreferred to as “weld buttering”. The weld is then post weld heat treated(PWHT) to assure proper tempering of the low alloy steel just below thedeposited nickel-based filler. Also, prior to PWHT, a weld clad using308L/309L or Alloy 52 clad may be applied along the inside diametersurface of the nozzle.

Once the nickel-based filler has been applied and the tempered conditionhas been generated for the nozzle (via the weld butter), a second weldis commonly required to join the buttered nozzle to the stainless steelsafe end. A nickel-based alloy such as Alloy 182, 82, or 52 may be usedto join these alloys. See FIG. 1 for an example of a nozzle-to-safe endjoint.

Due to the number of different materials and the reflectors generated atthe transition of the alloys, DMWs are also difficult to inspect. Whendamage is found, it is often isolated on the nozzle weld butter side(Alloy 182/82) of the joint which normally dictates that the entire weldis removed and then repaired. This can be both time consuming and costlyto a utility company.

Accordingly, there remains a need for a method that eliminates the DMWsentirely.

BRIEF SUMMARY OF THE INVENTION

This need is addressed by the present invention, which uses PM-HIP toconsolidate a nickel-based powder between two substrate materials (a lowalloy steel and a stainless steel) thereby eliminating the need for aweldment entirely.

According to one aspect of the invention, a method of joining dissimilarmetals includes the steps of providing a first part having a first alloycomposition; providing a second part having a second alloy compositiondifferent from the first part; connecting a containment structure to thefirst part; pouring a powder into the containment structure such thatthe powder is in contact with the first part; positioning a portion ofthe second part in the containment structure such that the second partcompresses the powder between the first and second parts; and performinghot isostatic pressing (HIP) to consolidate the powder and join thefirst and second parts together.

According to another aspect of the invention, a method of joiningdissimilar metals without welding includes the steps of providing afirst part having a first alloy composition; providing a second parthaving a second alloy composition different from the first part;connecting and sealing a containment structure to the first part;pouring a powder into the containment structure such that the powder isin contact with the first part; positioning a portion of the second partin the containment structure such that the second part compresses thepowder between the first and second parts and connecting and sealing thesecond part to the containment structure to form a complete assembly;and placing the complete assembly in a high pressure containment vesseland performing hot isostatic pressing (HIP) to consolidate the powderand join the first and second parts together.

According to an aspect of the invention, a method of joining dissimilarmetals includes the steps of providing a first part having a first alloycomposition; providing a second part having a second alloy compositiondifferent from the first part; connecting a containment structure to thefirst part; pouring a first powder capable of joining with the firstalloy into the containment structure such that the first powder is incontact with the first part; pouring a second powder capable of joiningwith the first powder and the second alloy such that the second powderis in contact with the first powder; positioning a portion of the secondpart in the containment structure and in contact with the second powdersuch that the second part compresses the first and second powdersbetween the first and second parts; and performing hot isostaticpressing (HIP) to consolidate the first and second powders and join thefirst and second parts together.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawing figures,in which:

FIG. 1 illustrates a nozzle-to-safe end weld joint;

FIG. 2 shows Charpy impact testing results for a first configurationusing PM-HIP nozzle-to-safe end joining;

FIG. 3 shows Charpy impact testing results for a second configurationusing PM-HIP nozzle-to-safe end joining;

FIG. 4 shows Charpy impact testing results for a third configurationusing PM-HIP nozzle-to-safe end joining;

FIG. 5 shows Charpy impact testing results for a fourth configurationusing PM-HIP nozzle-to-safe end joining;

FIG. 6 shows Charpy impact testing results for a fifth configurationusing PM-HIP nozzle-to-safe end joining;

FIG. 7 shows Charpy impact testing results for a sixth configurationusing PM-HIP nozzle-to-safe end joining;

FIG. 8 shows Charpy impact testing results for a seventh configurationusing PM-HIP nozzle-to-safe end joining;

FIG. 9 shows a microstructure of an Alloy 690 interlayer and 690-508interface associated with the seventh configuration of FIG. 8;

FIG. 10 shows a microstructure of an Alloy 82 interlayer and 82-508interface associated with the first configuration of FIG. 2;

FIG. 11 shows a microstructure of an Alloy 600M interlayer and 600M-508interface associated with the fifth configuration of FIG. 6; and

FIG. 12 is an exploded view of a nozzle-to-safe end joining by PM-HIP.

DETAILED DESCRIPTION OF THE INVENTION

Recent advances in powder metallurgy-hot isostatic pressing (PM-HIP)have demonstrated PM-HIP is capable of providing high quality low alloysteel, nickel-based alloys, and stainless steels for pressure retainingapplications that are readily inspectable, couple two different alloyssuch as bi-metallics for corrosion resistance or wear applications, andthat eliminates the need for DMWs.

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIGS. 2-11 illustratetest results for various configurations using the method of the currentinvention. The current invention uses powder metallurgy and hotisostatic pressing (PM-HIP) to consolidate a nickel-based powder betweentwo substrate materials (a low alloy steel and a stainless steel)thereby eliminating the need for a weldment entirely.

Several tests have been conducted to determine which PM-HIP processprovides the best properties (tensile, toughness, U-bends, hardness, andmicrostructure)—seven different joint configurations are discussedherein. These configurations have utilized one or more consolidatedpowders that bridge the gap between the two dissimilar materials and areshown in FIGS. 2-8.

The primary advantage of the PM-HIP joint versus a welded joint is thatmultiple alloys and weld joint angles are eliminated. Straight sidewalls are employed between the nickel-based powders and the twosubstrate materials (SA508 and austenitic stainless steel) resulting inan easily inspectable interface. The interface regions found indissimilar metal welds today exhibit multiple weld bevel angles andgeometric reflectors associated with welding.

The second advantage of the proposed PM-HIP application described hereinis that the use of 600M or other niobium bearing powders minimizes theformation of detrimental chromium carbides at the 508 interface region,thereby producing good properties, see FIG. 11.

Low Alloy Steel Side of Joint. The best configuration was determined tobe one that included an Alloy 600M consolidated powder between the twoalloys, FIG. 6. Detailed investigation of several of the jointconfigurations suggested that during HIP consolidation and processing,and subsequent heat treatment, carbon diffusion from the SA508 low alloysteel readily occurs at the interface boundary between the 508 materialand a nickel-based powder (FIGS. 9 and 10). The result is a distinctlayer of M23C6 carbides at the interface region which leads to lowductility and toughness in this area.

Utilization of the Alloy 600M powder as the joining media minimizes thedevelopment of the M23C6 carbides (M is usually Chromium). Stableniobium carbides are formed near the interface region, as shown in FIG.11. Nb-carbides do not appear to detrimentally affect tensile, hardness,bend, or toughness properties at the interface region. The premisebehind this effort is that niobium ties up the carbon before it is ableto join up with the chromium (which as noted earlier producesdetrimental properties).

While this discussion has focused on 600M powder, any Nb-bearingnickel-based alloy powder, such as alloys 600M, 713C, 718, 725, 52M andany other suitable powder, would serve the same purpose.

Stainless Steel Side of Joint. The above discussion has focused solelyon the SA508 side of the joint. For the stainless steel side of thejoint, it was shown that Alloy 690 or 690 Alloy (UNS NO6690), powderproduces an excellent joint. Unfortunately, 690 powder produces inferiorproperties when joined to the SA508 material as it does not containNb-carbides.

In terms of a single powder, it has been found that the best combinationis a 600M powder used with SA508 low alloy steel and 316L stainlesssteel. Another solution is to have a joint that includesSA508-to-600M-690-austenitic stainless steel. In other words,utilization of two powders would be employed: Alloy 600M and 690. Alloy600M provides a good transition to the 508 low alloy steel, while the690 provides a good transition to the austenitic stainless steel side ofthe joint.

Another example of where dissimilar welds are used is in componentswhere low alloy steel components are joined to nickel-based components.The method described above, may also be used to join these components.In this example, the nickel-based powders described above would still beused in a PM-HIP process as described above to eliminate dissimilarmetal welds.

In use, FIG. 12, a first part of a first alloy is joined to a secondpart of a second alloy during the manufacturing process. For clarity,the following discussion is related to a nozzle (first part) and safeend (second part) of dissimilar metals being joined; however, it shouldbe appreciated that the method is generally related to the joining ofdissimilar metal parts without the use of DMWs.

The method joins a nozzle 10 and a safe end 30 of dissimilar metals. Themethod uses a nozzle having a wall 12 with an inner wall surface 14, anouter wall surface 16, and a bore 18 defined by the inner wall surface14 and attaches a containment structure such as a shroud and/or can 20thereto via welding or other suitable attachment means. The can 20includes an inner sleeve 22 and an outer sleeve 24. The inner sleeve 22has an outside diameter “d1” similar to and/or equal to a diameter “d2”of the bore 18 to allow the inner sleeve 22 to be inserted into the bore18 and connected to the inner wall surface 14 of the nozzle 10. Theouter sleeve 24 has an inner diameter “d3” similar to and/or equal to anouter diameter “d4” of the wall 12 to allow the outer sleeve 24 to slideover a portion of outer wall surface 14 and be connected thereto andform a channel 26 between the inner and outer sleeves 22, 24,substantially equal to a thickness of the wall 12.

Once the channel 26 is formed, the appropriate alloy powder and/orpowders (i.e. two different alloy powders are used) may be poured intothe channel 26, such that the powder rests in the channel 26 and againstan end 28 of the nozzle 10. With the powder contained in the channel 26,a safe end 30 having a wall 32 with an inner wall surface 34, an outerwall surface 36, and a bore 38 defined by the inner wall surface 34 ispositioned in the channel 26 such that the wall 32 is positioned in thechannel 26 with the inner sleeve 22 positioned in the bore 38 and theouter sleeve 24 extends over a portion of the outer wall surface 36. Theinner sleeve 22 and outer sleeve 24 are connected to the safe end 30 inthe same manner as they are connected to the nozzle 10 to seal the can20 around the nozzle 10 and safe end 30. It should be understood thatthe powder contained in the channel is compressed between the end 28 ofthe nozzle 10 and an end 40 of the safe end 30.

With the nozzle 10, safe end 30, can 20, and powder all properlyassembled into assembly 100, a vacuum is pulled on the entire assembly100 through one or more access ports 44 (fill stems). The access ports44 are then crimped and welded shut. The assembly 100 is then placedinto a high pressure containment vessel and subjected to hot isostaticpressing (HIP) where elevated temperatures and gas pressures are used toconsolidate the powder and join the nozzle 10 to the safe end 30. Oncethe HIP process has finished, the can 12 is removed from the assemblyand any needed finish machining can be performed.

The foregoing has described a method of eliminating dissimilar metalwelds. All of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), and/or all of the stepsof any method or process so disclosed, may be combined in anycombination, except combinations where at least some of such featuresand/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings) may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

We claim:
 1. A method of joining a nozzle to a safe end without welding,comprising the steps of: (a) providing a nozzle having a first alloycomposition; (b) providing a safe end having a second alloy compositiondifferent from the nozzle; (c) connecting a containment structure to thenozzle, the containment structure having first and second spaced-apartsleeves, the first and second sleeves creating a channel therebetween,the channel having a width along its length equal to a thickness of awall of the nozzle and safe end; (d) pouring a first powder having athird alloy composition capable of joining with the first alloycomposition of the nozzle into the containment structure such that thefirst powder is in contact with the nozzle, wherein the first powder isa Niobium (Nb)-bearing nickel-based alloy powder; (e) pouring a secondpowder having a fourth alloy composition capable of joining with thethird alloy composition of the first powder and the second alloycomposition of the safe end such that the second powder is in contactwith the first powder, wherein the second powder is a 690 alloy powder;(f) positioning a portion of the safe end in the containment structureand in contact with the second powder such that the safe end compressesthe first and second powders between the nozzle and safe end; and (f)performing hot isostatic pressing (HIP) to consolidate the first andsecond powders and join the nozzle and safe end together.
 2. The methodaccording to claim 1 wherein the first sleeve of the containmentstructure is connected to an inner wall surface of a wall of the nozzleand the second sleeve of the containment structure is connected to anouter wall surface of the wall of the nozzle.
 3. The method according toclaim 1, wherein the first and second powders are compressed in thechannel between an end of the nozzle and an end of the safe end.
 4. Themethod according to claim 1, further including the step of connectingthe containment structure to the safe end.